Is it Time for the 6x45mm Cartridge?

The original caliber for the AR-15 wasn’t the .223/5.56, it was a slightly smaller cartridge. The .222 Special delivered the kind of performance that the designers wanted, which was basically a 50-grain bullet at under 3,000 feet per second.

The Army, trying to keep the AR away and keep the M14 in the running, kept moving the goalposts. Finally, they insisted that the bullet used had to penetrate a steel helmet at a distance farther than their own research had indicated soldiers fired on opponents. The special got stretched and boosted, until the 55-grain FMJ was at 3,100 fps.

And there it stood, until the mid-1980s, when the SS109 came about. That was intended for use against swarms of Soviet infantry in Western Europe. What, there never were swarms of Soviet infantry in Western Europe? Musta worked.

Seriously, the expectation was that the Soviets would roll West, and the NATO allies would be faced with Russian, East German, Polish and who knows who else mechanized infantry piling out of their BMDs, BMPs, and BTRs, lining up and assaulting the NATO positions. They expected to face lots of targets, and not only armed ones, but armored infantry. So, the push was for armor-piercing performance, leading to the SS109 and the later M-855, with a 10-grain steel penetrator tip inside.